Failure of a bus hoisting platform due to unexpected system behaviour

A bus hoisting platform showed an unexpected behaviour under asymmetric loading. In order to assess the internal restraints and joint forces which had not been considered properly in initial design, a numerical approach was chosen. With a multi-body simulation tool, the loads were calculated for different system configurations and loading situations. The identified loads were used for the stress analysis of individual components (analytical and numerical with FEM). The results revealed joint forces of a higher order than the total external load, which was a consequence of first the underestimated internal loads due to unapparent horizontal force components in the statically indeterminate system, and second, the implemented force control of the hydraulic actuators working in parallel. The correctly identified high internal loads explained the peculiar observations and lead to an insufficient fatigue strength of the welded structure.